Light sensitive motion detection devices typically comprise an optic or catoptric collecting assembly for maximizing signal gain and an electro optic sensor which receives a radiation signal. The sensor is coupled to electronic circuitry for detecting fluctuations in sensor output which result from changes in the radiation signal. Many passive infrared detection devices, such as the type used in security systems, monitor the movement of a body through an optical field of view based on emission of infrared radiation. Because persons and objects normally exhibit temperature differentials with respect to the surrounding background they emit infrared radiation levels which generally differ from the background levels. Consequently, their motion can be detected with electronic circuitry because they produce fluctuations in radiation intensity when passing through the optical field. By providing appropriate optical gain and signal discrimination small fluctuations in radiation intensity, e.g., corresponding to temperature differentials of less than 1.degree. C., can be detected. In order to enhance this sensitivity many passive infrared detection systems use precision mirrors or other collecting optics to focus radiation onto a parallel opposed dual pyroelectric detector comprising two spaced apart optical sensing elements. The pyroelectric detector will produce a defined signal from a moving target while cancelling common mode signals received simultaneously by both sensing elements. In addition, two stage optical filtering is sometimes required in order to narrow the frequency band of the optical response. This filtering eliminates background effects which may cause the intensity of the infrared radiation to fluctuate and induce a spurious response from the motion detection device. Signal processing may also be used to discriminate very weak signals caused by a moving target from unwanted signals caused by wind, clouds and precipitation.
The systems of lenses and mirrors which have been used in the past to provide high signal sensitivity in passive infrared motion detectors have provided a substantially planar and relatively narrow field of PG,4 view. As a result motion detectors are usually positioned to survey a substantially horizontal field of view. For example, a detection device may monitor motion in as many as 20 discrete horizontal zones wherein each zone forms a relatively small solid angle, e.g., on the order of seven millisteradians. This solid angle would only provide a cross sectional area of approximately 1.3 square meters at a distance of ten meters in front of the detector. Although motion detectors which sense movement in a plurality of discrete horizontal zones are capable of monitoring a relatively wide field of view, e.g. 60.degree., the vertical height of the zone is typically limited to approximately 1 meter at a distance of 15 meters from the detector.
At times it is desirable to monitor activity at close distances over relatively large surface areas that do not conform to a narrow horizontal field of view. For example, optical collecting devices used in the past for motion detection have not provided the panoramic field of view necessary for simultaneously monitoring an entire floor area in a room. This is because motion detection systems which monitor changes in infrared radiation over large solid angles, e.g., up to two pi steradians, have not been available.
In order to reliably detect motion throughout an entire room, e.g., with a ceiling mounted detector, the room must be divided into a large number of discrete detection zones. This requirement for a large number of zones, e.g., as many as one hundred zones in a room having a 3 meter ceiling height, limits the types of optical assemblies which can be used to collect radiation signals over a large solid angle. For example, segmented mirrors would require very complex optics in order to provide more than twenty zones. Fresnel lenses, while capable of providing a very large number of zones, cannot provide a wide field of view in two dimensions because a Fresnel lens cannot be curved along two transverse axes.
It is not believed that other mirror systems, such as, for example, deflecting mirror systems and multiple reflection mirror systems can provide a sufficient number of zones and an acceptable level of gain in order to reliably operate a ceiling mounted motion detection monitor. For a mirror system to provide a suitable number of zones there would have to be a complex arrangement of reflective surfaces and the reflectors would have to be positioned off of the optical axis. Otherwise the detector will obstruct radiation paths to the mirrors. Furthermore, significant signal loss may result from the multiple reflection mirror systems. Therefore it is desirable to have an improved optical assembly which focuses radiation from a large number of zones distributed over a large solid angle, e.g., two pi steradians, in order to provide a panoramic field of view for motion detection.